Light emitting diodes (LEDs) are semiconductor devices that generate light when electrical energy (e.g., current, voltage) is applied to the device. LED light output is proportional to the LED current, and thus, a current source is generally used to drive the LEDs. Advances in light emitting diodes (LEDs) have made LEDs very attractive for use in vehicles because of their long operation life, higher efficiency and low profile. For example, LEDs may be used in a headlamp system of a motor vehicle. A first set (e.g., string) of the LEDs are illuminated when the headlamp system is operated in a low beam operation mode, a second set (e.g., string) of the LEDs are illuminated when the headlamp system is operated in a high beam operation mode, and a third set (e.g., string) of the LEDs are illuminated when the headlamp system is operated in a daytime running lights operation mode.
The first, second, and third sets of LEDs may be arranged as two or more strings of LEDs connected in a parallel. The strings of LEDs are selectively illuminated in order to operate the headlamp system in a particular mode. Due to the wide unit-to-unit variation of LED forward voltage, parallel LED strings require a current limiter or other current regulator in series with each string in order to force current sharing amongst the strings. Resistors are commonly used as current limiters and series pass transistors are used as linear regulators in low power LED strings. However, the amount of power dissipated by the resistor or series pass transistor increases as the current increases in the LED string. Thus, resistors are not efficient current regulators for LEDs used in higher power applications such as motor vehicle headlamp systems. In these higher power applications, each LED string requires a low loss circuit, such as a switching power converter, for regulating the current. These switching current regulators are significantly more expensive than resistors or series pass transistors and thus implementing a switching regulator in each of multiple parallel strings increases the cost of the headlamp system.